1. Field of the Invention
The present invention relates to an intervertebral link device intended, in particular, for treatment of defects or pathological states of the vertebral column or of the vertebrae.
More particularly, the invention relates to an intervertebral stabilization device designed to hold at least two vertebrae in an appropriate relative position, e.g. for the purpose of correcting a patient's compacting of the vertebrae, scoliosis, lordosis, kyphosis, or intervertebral instability.
The invention can also be applied in the field of bone or joint links performed internally or externally.
2. Description of the Related Art
In the field of the preferred application to osteosynthesis of the spinal column, there exist numerous systems made up of link elements that are applied to the posterior face of the vertebrae and are fixed by screws implanted in the vertebral pedicles.
That type of assembly makes it possible to secure a plurality of vertebrae to one another, so as to obtain arthrodesis or bone fusion.
In the above technical field, a first category of devices is known comprising at least two link plates provided with slots and disposed longitudinally on either side of the spinous processes, and at least four fixing screws disposed in pairs on the pedicles on either side of two successive vertebrae. The threaded free ends of said screws pass through the slots in said plates and final fixing of the plates is provided, after adjustment, by nuts screwed onto the threaded ends.
The main advantage of such link plates lies in their rigidity. Nevertheless, it often happens that such rigidity can also constitute a drawback since it does not allow the surgeon to adapt the plates to the morphology of the vertebrae of a particular patient.
A second category of intervertebral link devices is also known, made up of a circular section rod fixed to bone anchor screws. An advantage of link rods lies in the fact that their circular section having a diameter lying in the range 4 mm to 7 mm makes it possible for the surgeon to shape them as required by using appropriate bending tools.
Nevertheless, it appears that an intervertebral stabilization device, whether of the plate type or of the rod type, once implemented, constitutes a system that is rigid, thereby applying mechanical stresses to the intervertebral joints adjacent to the joint being stabilized.
To remedy that problem, document EP 0 516 567 proposes an intervertebral stabilization device made in the form of a damper suitable for elastically withstanding axial compression and elongation. Such a stabilization device is capable of damping movement both in compression and in extension, thereby allowing the facets of the vertebrae to move asymmetrically relative to one another. Nevertheless, it turns out that such an intervertebral stabilization device designed to damp compression and extension movements does not give entire satisfaction.
It turns out to be necessary to have an intervertebral stabilization device that is suitable not only for damping axial movements in compression and traction, but also for damping lateral bending movements and bending and stretching movements in the antero-posterior plane.
Attempts have also been made to adapt the extent to which the mobility of the vertebrae to be stabilized is reduced as a function of each particular case, so as to leave a determined amount of lateral bending possible and a certain amount of axial compression and traction, the small amount of mobility that is allowed being predetermined to take up major stresses while still allowing micromovements to take place which are believed to favor bone fusion.